The present invention relates to a load control device for controlling a load such as a lamp of a vehicle.
Some of the related art load control devices include a triangular wave generation portion, a set voltage generation portion, a comparison portion and a driving control portion. In case a driving instruction signal to instruct driving of a load at a certain level corresponding to a fixed input is supplied, the triangular wave generation portion generates a triangular wave. The set voltage generation portion holds and generates a second set voltage set between the maximum voltage and the minimum voltage of the triangular wave. The comparison portion compares the triangular wave with the second set voltage. The driving control portion thus generates a driving control signal that changes its level with a constant frequency and duty based on the comparison result of the comparison portion.
In case a driving instruction signal to instruct stoppage of driving of a load at a certain level corresponding to a fixed input is supplied, the triangular wave generation portion generates a triangular wave. The set voltage generation portion holds and generates a third set voltage lower than the second set voltage. The comparison portion compares the triangular wave with the third set voltage. The driving control portion thus generates a driving control signal that changes its level with a constant frequency and duty based on the comparison result of the comparison portion.
In case a driving instruction signal that changes its level with a predetermined frequency and duty corresponding to a pulse input, the triangular wave generation portion generates a first set voltage set between the second set voltage and the third set voltage. The set voltage generation portion selectively generates the second set voltage or third set voltage in correspondence to the frequency and duty of the driving instruction signal. The comparison portion compares the first set voltage with the second set voltage or the third set voltage. The driving control portion thus generates a driving control signal that changes its level with the same frequency and duty as those of the driving control signal (for example, refer to JP-A-2001-148294 (claim 1, [A0019] to [A0053], FIGS. 1 to 3)).
In the above related load control device, the driving control portion generates and outputs a driving control signal that changes its level with certain frequency and duty even when the temperature changes. The ON resistance of a power MOSFET as a load driving element is substantially proportional to temperature and heat increases with temperature. Thus, it is necessary to perform heat dissipation design so that heat dissipation will be permitted at the expected maximum operating temperature. As a result, the device scale increases.
Also, the above load control device according to the related art uses a headlamp mounted on a vehicle such as a two-wheeled vehicle or a four-wheeled vehicle as a load. The headlamp mounted on a vehicle may be one including a low-beam lamp and a high-beam lamp attached to a single reflector or a single headlamp including a filament for low beams and a filament for high beams. Low beams are preferably turned ON so as not to cause glare on the eyes of the driver of a vehicle in front or an oncoming vehicle, if any, in night driving. High beams are preferably turned ON in the absence of a vehicle in front or an oncoming vehicle in night driving.
Some of the above vehicles have a feature called DRL (Daytime Running Light) that forcibly turns ON a headlamp in the daytime also in order to let pedestrians or oncoming cars recognize the presence of the vehicle and prevent possible traffic accidents. Some vehicles equipped with the DRL feature use low beams for DRL while others use high beams for DRL.
The related art load control device is composed of ICs and has a capacitor interposed therein as an external component between a connection terminal and a ground so as to set the frequency of a triangular wave generated by the triangular wave generation portion. In case the capacitor has shorted by some cause, the FET as a load driving element is maintained ON. As a result, in case the load is the headlamp, the headlamp is maintained ON with a 100% duty ratio.
With a vehicle using low beams for DRL, there are no particular problems even when the headlamp keeps lighting. The headlight lighting state ensures safety of the people on the vehicle, pedestrians and oncoming vehicles so that the lighting state is rather favorable from the viewpoint of a fail-safe design. With a vehicle using high beams for DRL, the headlight lighting state is maintained with a 100% duty ratio. This could cause glare with respect to the driver of a vehicle in front or an oncoming vehicle which leads to a traffic accident.
This advantage could be common to any device in general that controls a load based on a generated triangular wave signal.